Hi-Fi amplifiers from decades ago often had selectors for different phono signal levels. That was because the output from a turntable depends on the cartridge design: moving magnet (MM) cartridges have much hotter outputs compared to moving coil (MC) designs. To provide a useful volume range, the preamplifier needs to use correspondingly different gain for these types.
Plugging a line-level output into a phono jack is a pretty bad idea, though. Line level can vary but in the worst case it can be over a volt, whereas a MC input expects a 250 microvolt signal. A MC input may also have a rather delicate transformer attached, and it could have been negatively affected.